Antistatically protected nonwoven polyolefin sheet

ABSTRACT

Nonwoven sheets comprised of film-fibril elements of an olefin polymer and bearing at least 0.1 percent by weight, preferably 0.3 to 0.4 percent by weight of a non-rewettable finish defined by the formula: MnR3-nPO4 where M is selected from the group consisting of lithium, sodium, potassium, and ammonium ions, R represents an alkyl group containing 3 to 5 carbon atoms, and n is selected from the integers 1 and 2.

United States Paten n91 McMillin fl 1 1111 3,821,021 1 51 June 28, 1974ANTISTATICALLY PROTECTED NONWOVEN POLYOLEFIN SHEET [75] Inventor: CarlKenneth McMillin,

Wilmington, Del.

[22] Filed: Feb. 29, 1972 [2]] App]. No.: 230,448

[52] U.S.Cl..' 117/1355, 117/1388 E,

} 117/139.5CQ [51] Int. Cl C09d 3/00 [58] Field of Search ..117/138.8 E,135.5,

[56] References Cited UNITED STATES PATENTS 2,413,428 12/1946 Billings117/1395 X 2/1965 Steuber l56/62.2X

3,442,740 5/1969 David 156/181 3,478,141 11/1969 Dempsey et a1. 156/166X3,658,573 4/1972 Guestaux etal. 117/1388 X 3,703,588 11/1972 Saito et alll7/l38.8 X

Primary Examiner-Michael Sofocleous Assistant Examiner-Bernard D.Pianalto [57] ABSTRACT 4 Claims, No Drawings ANTISTATICALLY PROTECTEDNONWOVEN POLYOLEFIN SHEET BACKGROUND OF THEINVENTION Methods areavailable for preparing nonwoven polyolefin sheets which exhibit theability to function as a barrier to liquid water while still permittingpassage of water vapor, by fibrillating an oriented polymeric film toform a network and then laminating together several layers withfibrilsoriented in different directions in the various layers. In onemethod, a solution of polymer is flash-spun at a temperature above theboiling point of the solvent and at a high pressure intoa low pressurearea, whereupon a three-dimensional network of filmfibrils forms at thespinneret. The network is spread by means of a baffle and is thencollected in multidirectional, overlapping, and intersecting arrangementon a moving belt. The sheet may be consolidated by passing it throughthe nip of .a pair of cold rolls. A hot embossing process for improvingthe delamination and abrasion resistance of such sheets while retainingthe softenability of the sheet is also known. Such nonwoven productsexhibit desirable combinations of softness, drape, breatheability, etc.,which has led to their acceptance for use in disposable or limited-usegarments, as protective wrapping materials, drapes and curtains, etc.

The ability to function as a barrier to liquid water while stillpermitting passage of water vapor results from the film-fibril structureof the nonwoven sheet which permits passage of water (and other gaseous)vapor through the tortuous paths from one surface of the sheet to theopposite surface provided by the microscopic channels between therelatively close packed film-fibril elements, while liquid water isprecluded from penetrating these same channels due to their small sizeand the hydrophobic character of the polyoletinpolymer surface. Therelative freedom for passage of vapor through the nonwoven sheets can bevaried over a substantial range, depending on how tightly compacted (howhighly'calendered or embossed) the sheets may be, whether subsequent tobonding or embossing the sheet has been mechanically worked to loosen upthe film-fibril structure between bond points, and: on the total basisweight of the sheet etc. In any event, the water barrier propertycontinues to be exhibited, provided the nonwoven sheet remains free fromgross holes as might be deliberately introduced.

Such nonwoven fibrous sheets having vapor transmission and liquid: waterbarrier properties have obvious potential utility in a multitude ofapplications, e.g., disposable, protective garments such as surgeonsgowns; protective wrap for various commodities such as carpets, fiberbales, lumber, grain, etc.'; sterile packaging permeable tosterilizinggases but impermeable to bacteriaor liquids; consumer products suchaswind breakers, rainwear, sleeping bag liners, etc. However, thewellknownlpropensity ofpolyolefin articles to accumulate staticchargesisadisadvantage. Unfortunately, the conventionaltechnique ofsimply applying antistat finishes to the nonwoven sheets does notordinarily lead to a satisfactory. solution, since eventhough certain ofsuch finishes will: adequately suppress static charge generation, theysimultaneously confer a hydrophilic, rewettable character: to thepolyolefin nonwovensheet,

and thus completely destroy its liquid water barrier property.

SUMMARY OF THE INVENTION According to the present invention, we havediscovered a limited class of agents having a balance of high antistatactivity and moderate aqueous surfactant power such that they may beapplied as a finish onto nonwoven sheets composed of polyolefinfilm-fibril elements to confer antistatprotection without destruction ofthe sheets inherent liquid water barrier (hydrophobic) properties. i i

The present invention concerns a nonwoven sheet comprised of film-fibrilelements of an olefin polymer which sheet bears at least 0.1 percent byweight, preferably at least 0.3 percent by weight of a non-rewettablefinish agent defined by the formula: M,,R;,-,,PO where M is selectedfrom the group consisting of lithium, sodium, potassium, and ammoniumions, R represents an alkyl group containing 3 to 5 carbon atoms, and nis selected from the integers l and 2. The preferred agents are thosewhere M represents the potassium ion, and the most highly preferredfinish is a mixture of approximately equimolar quantities of the twoagents potassium dibutyl phosphate and dipotassium butyl phosphate. Thepreferred polyolefin is linear polyethylene. It is preferred that atleast 0.3 percent but no more than about 2 percent by weight of agent bepresent on the sheet. Excessive amounts of agent are not onlyunwarranted for economic reasons but are also undesirable because thewater barrier properties of the sheet are adversely affected. 1 i

The antistat finish of this invention may be applied to the polyolefinnonwoven sheets by immersing the sheet in an aqueous solution of theagent; by kiss-roll, gravure roll, or similar application of a coatingof an aqueous solution of the agent; or by spraying an aqueous solutionof the agent onto the surface of the sheet. For those agents whose watersolutions have surface tensions appreciably in excess of thecriticalsurface tension of the polyolefin substrate, it is advantageous to lowerthe surface tension of the applied solution by addition of a thirdcomponent such as a non-rewettable surfactant or a volatile alcohol.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The nonwoven fibrous sheetsuseful for this invention may be prepared from any crystalline olefinpolymer but linear (high density) polyethylene is the preferredpolyolefin; They are composed of film-fibril elements having a: surfacearea greater than 2 meter lgm. and may be produced by a flashextrusionmethod as dis closed in US. Pat. No. 3,169,899. The nonwoven sheets maybe cold consolidated as described in said US. Pat. No. 3,169,899, orpoint embossed as disclosed in US. Pat. No. 3,478,141, or surface bondedtype as disclosed inIlS. Pat. No. 3,442,740.

The operable agents for the finish/coatings of the present inventionpossess a critical balance of high antistat activity and moderateaqueous surfactant power. The mono and dialkyl phosphates wherein thealkyl group contains 3 to 5 carbon'ato'ms (i.e., n-propyl, isopropyl,n-butyl, isobutyl, secondary butyl, tertiary butyl, n-amyl, andthevarious C alkyl isomers) have been found to possess the requiredlevelofaqueous surfactant power to be useful in the present invention.

Although we do not wish to be bound by any theory, a working hypothesisconsistent with the present invention is that the antistat agent mustexhibit a certain level of aqueous surfactant power. If the antistat istoo good a surfactant, then when the coated nonwoven sheet subsequentlycomes adventitiously in contact with water, a quantity of antistat willdissolve into the water to form a solution whose surface tension wouldbe lower than the critical surface tension of the polyolefin substrate,and this solution will therefore wet the nonwoven sheet, penetrate themicroscopic capillary channels between the individual film-fibrilelements, and thereby destroy theliquid water barrier property of thesheet. This critical surfactant activity limit appears to be exceededwhen the alkyl group of the alkyl phosphate antistat contains six ormore carbon atoms. On the other hand, some minimum level of surfactantpower is required of the antistat simply in order that it may be appliedonto the nonwoven sheet (either by immersion, surface coating, spraying,etc.) from aqueous solution. The requirement is believed to be that thesurface tension of the antistat solution should be such that thesolution almost wets the nonwoven sheet. For example, in application ofthe antistat finish to linear polyethylene, the antistat solution shouldhave a surface tension of about 37 i 3 dynes/cm. A 0.2 1 percent aqueoussolution of potassium amyl phosphate or a l-2 percent solution ofpotassium butyl phosphate will satisfy this requirement directly.Potassium propyl phosphate, however, is not powerful enough a surfactantto meet this surface tension requirement at reasonable concentrations(less than 2 percent) in pure water, but it may successfully be appliedto linear polyethylene from a 1 percent solution in a l7-volume percentisopropanol aqueous solvent (surface tension equal 35 dynes/cm.). Inlike fashion, sodium butyl phosphate and ammonium butyl phosphate areboth somewhat poorer surfactants than potassium butyl phosphate so thattheir water solutions at less than 2 percent concentration fail to meetthe surface tension requirement; but they too may successfully beapplied to linear polyethylene from an alcohol/water solvent. Similarly,potassium butyl phosphate aqueous solutions at concentrations less than1 percent have surface tensions which are too high (greater than 40dynes/cm.), but such solutions can be made operable for coating linearpolyethylene nonwoven sheets by adding a small quantity (e.g., 0.05 to0.] percent) of a non-rewettable surface active agent, such as IgepalCO-880 or CO-890 (registered trademark of GAF Corporation's nonionicnonylphenoxy polyoxyethylene ethanol surfactants). These materials aretermed non-rewettable here in the sense that a linear polyethylenenonwoven sheet coated with these surfactants is not penetrated by a dropof water subsequently placed on the surface of the treated sheet.

The antistat finishes of the present invention may comprise a singleagent of the formula: M,,R ,,PO (as previously defined) or it may be amixture of such agents, e.g., potassium butyl phosphate plus potassiumisoamyl phosphate. Frequently mixtures are obtained directly incommercial operations. M may be chosen to be lithium, sodium, potassium,or ammonium ion but the potassium salts are preferred. The agent mustcontain at least one R group and at least one M group. It is to be notedthat the finishes employed in this invention are not hydrophobicmaterials like the water repellent coatings conventionally applied tovarious woven and nonwoven fabrics in order to render them waterproof,"but are rather a very restricted group of antistat materials which maybe applied to polyolefin film-fibril nonwoven sheets without destroyingtheir inherent hydrophobic liquid water barrier property.

In the Examples which follow, all samples are prepared on a laboratoryscale by immersing a portion of the nonwoven sheet material in anaqueous bath of the specified composition. The sheet is removed from thebath and passed through the nip of a pair of elastomcric squeeze rolls2% inch (5.4 cm.) in diameter of t 5 Shore A Durometer hardness operatedat a nip pressure of 4-8 pli to remove excess solution from the surfaceof the sheet. The wet sheet is then air dried followed by an additionaltwo minutes drying period at l 10C. The antistat protection provided bythe finish is determined by submitting the sample previously conditionedat least 24-hours at the indicated temperature and relative humidity totest NFPA (National Fire Protection Association) Code 56A, Section25433; paragraph A, part 3. The result is reported as Log R" with valuesof 11 (75F. and 50 percent'RH.) being passing and lower valuespreferred. The water barrier performance of the sample is measured bytwo tests the hydrostatic head test ASTM D-583, paragraph 53A, MethodII, results reported in inches" (cms.) with high values preferred; andthe rain penetration test ASTM D-583, paragraph 32-37, results reportedin grams with lower values preferred.

EXAMPLE I This example illustrates the effect of varying the size of thealkyl groups of the phosphate antistat agents. Samples of nonwoven sheetcomprising film-fibril elements of linear polyethylene are prepared bythe pro cess of U.S. Pat. No. 3,169,899 at a basis weight ofapproximately 2.2 oz./yd. (75 gms./m. and subsequently bonded by theprocess of U.S. Pat. No. 3,442,740. These samples are treated with 0.5weight percent aqueous solutions of various antistats as indicated inTable I. These antistats are all mixtures (approximately equimolar) ofmonopotassium dialkyl and dipotassium monoalkyl phosphate salts with thealkyl group indicated. At the 0.5 percent concentrations employed,potassium butyl phosphate, having the lowest surfactant power, can besuccessfully applied to the linear polyethylene substrate only by addinga small quantityof a surface tension depressant, in this case IgepalCO-890. This latter material by itself provides essentially no antistatprotection for the linear polyethylene substrate.

TABLE I Rain Penetration Log R Run Antistut Hydrostatic Head 2 .12 min.(ol cm/Z min) 75F./55Z RH l-A None 60 in. 152 cm.) 0 g. Y l4 l-BPotassium butyl phosphate 50 in. I27 cm.) 0 g. 8.0 l-C Potassium amylphosphate 47in. H9 cm.) 0.2 g. 8.l

TABLE 199901115 1 Rain Penetration Log R 75F./557z RH Run AntistatHydrostatic Head 2 0.12 min. (61 cm/2 min) l-D Potassium hexyl phosphate31 in. 79 cm.) 3.6 g. 8.0 l'E Potassium octyl phosphate 4 in. 10 cm.)4.0 g. 7.5

Plus 0.11% lgepal" (-1190 Note that although all treated samples exhibitexcellent antistat protection (values of Log R less than 1 l the goodliquid water barrier properties of the samples are lost when the size ofthe alkyl group is increased beyond the C (amyl) limit of the presentinvention.

EXAMPLE II This example illustrates the effect of changing the quantityof antistat agent applied as a finish to the nonwoven sheet. Samples ofnonwoven sheet comprising film-fibril elements of linear polyethyleneprepared by the process of U.S. Pat. No. 3,169,899 at a basis weight ofapproximately 1.3 oz./yd. (44.2 gms./m. and subsequently embossed on oneside with rows of pointbonds (rib pattern) and surface-embossed on theopposite side with a linen pattern" (as in Examples VI and VII of U.S.Pat. No. 3,427,376) are treated with alkali metal or ammonium alkylphosphate salts represent an approximately equimolar mixture of a saltcon taining one alkyl group with twoalkali metal or ammonium ions and asalt containing two alkyl groups with one alkali metal or ammoniumion.

EXAMPLE 1111 This example illustrates the effect of various candidateantistat finishes, both within and outside the limitation of the presentinvention. All candidates are appliedfrorn a solution at the specifiedconcentration, as indicated in Table 111, in a solvent comprising 17volume percent isopropanol in water such that in each case the surfacetension of the solution is low enough to insure good wetting duringapplication to the same linear polyethylene nonwoven substrate employedin Example 11. Note that only the alkali metal and ammonium alkylphosphates furnish good antistat protection (Runs 3A through 31 versus3.1 and 3K), that presence of an alkyl group is essential (Run 3L), andthat common quaternary alkyl. ammonium halides are ineffective (Runs 3Mthrough 3R V .TABLE 111 Antistat Concentration Linen Side/Rib side 3-APotassium propyl phosphate 0.5 wt. 7: 13/1 1.9 3-B do. 1.0 l0/9.5 3-Cdo. 2.0 9.0/8.5 3-D do. 3.0 7.6/7.3 3-E Potassium butyl phosphate 0.59.9l0 0/9.0-9 5 3 F do. 1.0 8.99.0/8.4-8 7 13-6 do. 1.7 7.8/7.3 3-HSodium butyl phosphate 1.0 9.0/9.0 3-1 Ammonium butyl phosphate ,1 .0'9.0/9.0 3-.l Diethanolamine butyl phosphate 1.0 12-13/12-13 3-KGuanidine butyl phosphate 1.0 13-14/13-14 3-L Tri-potassium phosphate3.0 l4l l4 3-M Tetramethyl ammonium chloride 7 .1.10 13/ 13 "3-NTetramethyl ammonium bromide 1,54 13/ 13 3-0 Tetraethyl ammoniumchloride 1.66 13/ 13 34" Tetraethyl ammonium bromide 2.10 13/ 13 3-QTetrabutyl ammonium bromide 3.22 12/ 1 1 3-R Tetrabutyl ammonium iodide(sat. solution) 12/ 12 ings achieved with the more concentratedsolutions do EXAMPLE 1V lead to small incremental improvements inantistat protection (lower values of Log R), the excellent waterProducts of the present invention are prepared as inbarrier property ofthe sheets is not destroyed.

dicated at Table IV, employing the same linear polyeth- ABL Cone. of LogR Potassium Hydrostatic Rain Pene. 70F (21C.)/ Run Butyl Phosphate Head2 ft/2 min. 55% RH 2-A 1.0 wt. 41 in. (104 cm.) 0 g. 8.8 2-B 1.5 41 in.(104 cm.) 0 8.1 2-C 2.0 39 in; 99 cm.) 0 7.7 2-D 3.0 41 in. (104 cm.) 07.0 2-E 4.0 in. (102 cm.) 0 6.8 ZF None 0 l4.0

40-50 in. (102-127 cm.)

In this example and inExamples 111 and IV which folylene nonwovensubstrate employed in Example 11. The

low, it should be understood that each of the indicated quantity ofantistat finish on the treated sheet may be determined either bystandard analytical techniques (e.g., phosphorous or potassium analysis,using either chemical or spectroscopic techniques) or simply bycalculating the amount of antistat contained in the quantity of treatingsolution picked up by the nonwoven sheet prior to drying, since thevolatility of the antistat salts is very low.

The antistats are dissolved in water to the indicated weight percent,the sheet samples immersed in the solution and subsequently run throughsqueeze rolls at the indicated pressures, at which point the percentsolution pickup is immediately determined by measuring weight gain, andthe sample is then dried. The pickup of anti stat, based on the driedsheet, is computed from the sowhile permitting passage of water vapor,comprised of film-fibril elements having a surface area greater than 2meter /gm, said sheet bearing from 0.1 percent to 2 percent by weight ofa finish defined by the formula: M,,R ,,PO where M is selected from thegroup consisting of lithium, sodium, potassium, and ammonium ions, Rrepresents an alkyl group containing 3 to 5 carbon atoms, and n isselected from the integers l and 2.

2. The sheet of claim 1 bearing from 0.3 percent to 2 percent by weightof said finish.

3. The product of claim 1 wherein the finish is a mixture ofapproximately equimolar quantities of potassium dibutyl phosphate anddipotassium butyl phoslution pickup and antistat concentration. W P ITABLE IV Run Antistat Conc. Squeeze Roll Pressure Solution PickupAntistat on Sheet Log R 4A Potassium Propyl Phosphate 2.5% 4.3 p11 20.5%0.51% 8.8 43 o. 5.0 do. 262 1.31 8.0 4C Potassium Butyl Phosphate 1.58.5 26. 0.39 7.9 4D do. do. do. 21. 0.32 7.9 4E do. .0 do. 30.5 0.61 774E do. do. do. 27.6 0.55 7.8 46 do. do. do. 25.3 0.51 7.8 4H do. 3.0 do.30.5 0.91 7.35 41 do. do. 4.3 31.5 0.94 7.3 4] do. do. do. 34.3 1.03 6.74K Potassium Butyl Phosphate 0.5 do. 31. 0.16 7.8

lgepal CO-890 0.1 4L do. do. 8.5 25. 0.12 7.8

4. The product of claim 3 wherein the polyolefin is linear polyethylene.

2. The sheet of claim 1 bearing from 0.3 percent to 2 percent by weightof said finish.
 3. The product of claim 1 wherein the finish is amixture of approximately equimolar quantities of potassium dibutylphosphate and dipotassium butyl phosphate.
 4. The product of claim 3wherein the polyolefin is linear polyethylene.